Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires gas ion bombardment of a target having a face formed of a material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target material to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated by use of a cooling fluid typically circulated beneath or around a heat conducting backing plate that is positioned in heat exchange relation with the target.
The target forms a part of a cathode assembly which together with an anode is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target materials traverse the evacuated enclosure and deposit as a thin film on the desired substrate that is normally located proximate the anode.
In addition to the use of an electrical field, increasing sputtering rates have been achieved by the concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent the target surface thereby increasing the number of electron-gas atom collisions in the area to produce an increase in the number of positively charged gas ions in the region that strike the target to dislodge the target material. Accordingly, the target material becomes eroded (i.e., consumed for subsequent deposition on the substrate) in a generally annular section of the target face, known as the target raceway.
In typical target cathode assemblies, the target is attached to a nonmagnetic backing plate. The backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target. Magnets are typically arranged beneath the backing plate in well-known disposition in order to form the above-noted magnetic field in the form of a loop or tunnel extending around the exposed face of the target.
In order to achieve good thermal and electrical contact between the target and backing plate, these members are commonly attached to each other by means of soldering, brazing, diffusion bonding, clamping and by epoxy cement and the like. However, sputter target assemblies bonded by these methods can bow or bend at high sputtering temperatures, especially when a large difference exists between the coefficients of thermal expansion for the target and backing plate. In many cases, in order to minimize bowing and bending of the plates at high sputtering conditions, targets and backing plates having similar thermal expansion coefficients are bonded together.
In many situations, the targets and backing plates can be diffusion bonded. However, with regard specifically to diffusion bonding that is carried out in HIP cans, HIP cans may leak during the HIP process thereby preventing or diminishing metal target bonding to a backing plate due to contamination or residual air in the HIP can.
Additionally, the high temperatures that can be associated with some of the above disclosed bonding methods can result in undesirable grain growth in the target metal.
The present invention pertains to a low temperature bonding method which provides for the consolidation of sputter target/backing plate assemblies. These sputter target/backing plate assemblies are comprised of targets and backing plates having dissimilar thermal expansion coefficients. Although the target and backing plate have dissimilar thermal expansion coefficients, the method allows these unconventional assemblies to be bonded together and used at high sputtering temperatures without significant bowing or bending. As a result, these sputter target assemblies are able to utilize backing plates having higher thermal and lower electrical conductivity than the conventional backing plates normally associated with a specified target metal. Lastly, this method allows for the target assembly to minimize grain growth in the target metal due to consolidation at a low temperature.